>>22218642Red and Green were released in February 1996 (originally scheduled for December 1995).
Blue was released in October 1996 exclusively to CoroCoro subscribers as a version to fix bugs; it was later released to retail in October 1999.
Yellow was released in September 1998, 2 weeks before Red and Blue were released in North America. This was after Game Freak postponed the original release date of Gold and Silver (March 1998), meaning that they wanted to buy time and cash in on the anime's popularity (the anime's first episode was aired in April 1997).
Ishihara has confirmed that he thought that Gold and Silver would be Game Freak's last Pokémon project.